Sn—Pb solder has conventionally been used in joints in electronic equipment. However in recent years the use of conductive paste is under consideration in response to demands for lead-free solder. Furthermore, devices employing conductive resin have become widely used in recent years.
The conductive pastes and conductive resins used in these applications are pastes, wherein conductive particles and various types of resins are kneaded together, and compacts formed by hardening the pastes. The properties required of conductive particles are high electrical conductivity of the particles themselves, low resistivity of the compact obtained by kneading the conductive particles with the resin, high resistance to migration, and superior weather resistance, and the like. Metal powder and carbon powder are currently employed as conductive particles.
However, of the metal powders, the precious metals have high electrical conductivity and low resistivity, but are expensive. Moreover, base metals as represented by nickel and copper and the like are inexpensive and have high electrical conductivity. However they have inferior weather resistance, and when used kneaded with resins to obtain conductive pastes and conductive resins, suffer from increased resistivity with long-term use. On the other hand, carbon powder is inexpensive and has high weather resistance. However it has low electrical conductivity, and high resistivity when kneaded with resin.
A powder wherein the surface of nickel particles or copper particles is coated with a precious metal such as Ag and the like has been proposed (Japanese Patent Unexamined Publication No. 2002-025345, and Japanese Patent Unexamined Publication No. 2002-075057) as a method of resolving these problems. The precious metal coating on the nickel particles or copper particles of these powders provides improvements in terms of properties, however they are expensive in cost. In particular, silver-coated powder is unsuitable for use in environments where resistance to migration is required.
Furthermore, attempts have been made to alter the surface shape of the nickel particles and the like, for example, by formation of small hemispherical protrusions on the surface, in order to decrease resistivity when kneaded with resin (Japanese Patent Unexamined Publication No. 2001-043734 and U.S. Pat. No. 5,378,407 specification). However, as the inferior weather resistance has not been improved, stability in long-term use cannot be considered to have improved. Based on this situation, provision of conductive particles that are inexpensive, have superior weather resistance, low resistivity when kneaded with resin, and stability when used in the long-term, is desired.